Harumi Chouno is introduced as the private tutor for Taira Yuuichi, a role in which she is perceived as a calm and professional oasis for her stressed student. On the surface, she is a sweet, law-abiding, and conservatively dressed research assistant at Jiai University. However, this public persona conceals a profound internal struggle, as she suffers from dissociative identity disorder. For reasons left unexplained, Harumi has formed a second, dominant personality she calls Maria. In stark contrast to the reserved Harumi, Maria is an outspoken prostitute who uses a different dialect, wears wigs and a fake mole, and dresses in flashy, revealing fashions. The two personalities communicate by leaving each other messages on the answering machine in Harumis apartment, functioning as separate entities who share the same life and body. While they are often at odds, Maria initially believes she is acting on Harumis deepest repressed desires, acting as a darker counterpart to the oasis Harumi provides for others.

The central conflict of Harumis story begins when a male colleague from the university proposes marriage, an offer she accepts. Terrified of her fiancé discovering her alternate life, Harumi decides to kill off the Maria personality. She quits Marias sex work and throws away all of Marias clothing and wigs, believing she can start a new, honest life as only herself. This action, however, triggers a violent backlash. Maria begins calling the apartment, demanding to be set free and insisting that Harumi is the fake personality. Harumis grip on reality rapidly deteriorates; she experiences blackouts and realizes Maria is seizing control of their body against her will. Her psychiatrist advises her to confess everything to her fiancé, but she is unable to. In desperation, she visits the hospitalised Yuuichi, who remarks that she needs a doctor more than he does. The internal war for dominance culminates in a public, one-sided battle where Harumi appears to be physically fighting and arguing with herself in the street at night, pulling her own hair and speaking in both personas. It is at this moment of extreme psychological crisis that Lil Slugger appears and strikes her, making her the assailants fourth victim.

Her motivations are driven by a desperate need for control and the desire for a stable, socially acceptable future through marriage. She wishes to be seen as "clean" and normal, which puts her in direct opposition to the uninhibited, sexual Maria. Her key relationship is with her alternate personality, Maria, creating a singular and antagonistic internal tie. Externally, her relationship with her fiancé is based on a lie she cannot maintain, and her connection to her student, Yuuichi, highlights her role as a caretaker who is herself in desperate need of help. The character undergoes a sharp development from a functional, albeit fractured, individual to a woman in a state of complete psychological collapse, ultimately being physically and metaphorically broken by the series' central figure. Harumi possesses no supernatural abilities. Her only notable trait is her dissociative identity disorder, a psychological condition that allows her two distinct personas, Harumi and Maria, to operate almost independently. Her surname contains "Cho," a part of the Japanese word for butterfly, a motif of transformation and identity that echoes the central themes of her story.